1. Field of the Invention
The invention relates generally to a sub assembly for electrically insulating or isolating components of a drill string, i.e., tubing, drill pipe, drill collars and the like. More particularly, but not by way of limitation, it relates to an improved insulative sub assembly utilizing a high performance plastic in combination with steel connecting components to provide electrical insulation while maintaining drill string requirements as to torque, tensile and compressive forces through the sub assembly.
2. Description of the Prior Art
The prior art has included numerous methods and devices for causing a gap or insulation in components down along a drill string in order to develop electrical currents thereacross for transmission to the surface. Such transmission is utilized during MWD (measurement while drilling) data transmission uphole and it is used variously during logging operations for conveying different types of intelligence or modulation components to the surface for reception and processing. An early form of downhole insulation as taught by U.S. Pat. No. 2,354,887, a well signaling system, utilizes a coating of Bakelite insulation at a position downhole on a drill string whereupon, utilizing a pair of insulated electrodes, transmission energy sets up a radiating electric field that can be detected at the earth surface. A similar teaching is disclosed in U.S. Pat. No. 4,160,970 where the drill string itself is used to form an antenna as a counterpoise connection to the surrounding earth completes the sensing circuit for the receiver equipment.
U.S. Pat. No. 4,691,203 teaches a mode transducer that can be interconnected in the drill string. The mode transducer is formed from serial conductive sleeves separated by an insulating sleeve made of hard-anodized aluminum alloy. U.S. Pat. No. 4,348,672 teaches another form of drill collar gap sub that utilizes an insulative liner facing all joinder surfaces between successive pipe sections. This insulated gap sub consists of a first annular sub member that is threadedly received into a second annular sub member as connecting interfaces (including threads) are lined with an insulating dielectric material.
U.S. Pat. No. 4,387,372 utilizes an insulated point gap assembly for a toroidal coupled telemetry system. U.S. Pat. No. 4,496,174 discloses another form of drill collar gap sub wherein facing structure at joinder points of the drill string is insulated. Thus, an insulative material is applied on all threaded facings and annular washers coated with insulative material are used to further seal abutting interconnections between successive sub members. Finally, U.S. Pat. No. 4,684,946 teaches a drill collar having an insulating sheath around which is disposed a cylindrical metal sleeve for propagation of transmission energy. Sensor and transmitter equipment packages are contained in the drill collar interior and electrical connection via wire lead is made to the outer metal sleeve. In any of the various type gap subs which enable radio transmission it is essential to provide an insulation gap in the drill collar or such notwithstanding the severe environmental loading that will be encountered.